This invention relates to a method and system for calibrating wideband direction finding devices. More particularly, the invention relates to a calibration method utilizing a data frame technique to sample the entire passband of an antenna array.
In the field of radio communications, direction-finding (DF) equipment is essentially an antenna-receiver combination that determines the coordinates of a radio signal relative to the DF location. DF equipment enables one to locate or monitor various radio signal sources, stationary or otherwise, by determining the direction of arrival (DOA) and/or angle of arrival (AOA) relative to the ground of the emitted radio signal from a source to the DF receiver. DF equipment has many terrestrial and aerospace applications. Such applications may require the ability to determine the coordinates of radio source over a broad range of broadband frequencies. For instance, wideband DF equipment may scan anywhere from a range of a few MHz to several thousand MHz. As a result, wideband DF equipment has been very useful in various applications which track a wide range of signal frequencies.
The calibration of wideband DF equipment has been a challenge. External factors, such as weather conditions and location, alter the calibration factors of DF equipment. The calibration method utilized should be accurate within an acceptable tolerance over a wide frequency range. The DF equipment scans the entire range of frequencies and stores a series of calibration factors for the entire range.
Usually, DF equipment has an antenna portion with at least two antenna elements. The antenna portion may be rotatable or stationary. The rotatability allows for the determination of the direction of arrival of the signal. For a specific frequency range, specific frequencies are sampled over time. This time-sampled data is then transformed into the frequency domain using the Fast Fourier Transform (FFT). Subsequent measurements are obtained by retuning the DF equipment to scan a different frequency range. For each frequency range, any amplitude and/or phase differences between signals received by the antenna elements are then processed to determine the DOA and AOA coordinates of the signals.
To calibrate the DF equipment, calibration measurements have to be made to obtain calibration factors. These calibration factors are to be taken into account when performing DF measurements to allow for inconsistencies introduced in the DF measurements by the DF equipment itself. A noise source can be used as the signal source for the DF equipment when conducting calibration measurements. These calibration factors are measured for each frequency range to be scanned by the DF equipment.
Tkalcevic, in U.S. Pat. No. 5,056,051, discloses an apparatus and method of calibrating a DF processing system. Tkalcevic measures the phase and amplitude responses of received signals over a narrow passband. The passband is divided into a number of segments called xe2x80x9cbinsxe2x80x9d, each bin is associated with a component of the FFT of the signal. Since each bin includes phase and amplitude information, both sets of values may be averaged to produce an average along the passband calibrated.
The Tkalcevic method is not practical for wideband usage because several signals might be present in a passband that may be wider than necessary. DF tuners have a maximum frequency range which they may cover. Retuning of the DF tuners is required each time a new frequency range is scanned by the DF system. There is a need for a calibration method that minimizes the amount of tuning required for the entire range of frequencies measured by a wideband DF system.
The present invention seeks to provide a method of calibrating a wideband DF system using a frame sampling method.
The invention is a method of calibrating the sensor system of a wideband direction finder using a noise source. The sensor system includes at least two wideband tuners, and an analog/digital (A/D) converter connected to each wideband tuner. The calibration method involves selecting a common set of centre frequencies for each of the wideband tuners. The wideband tuners receive a signal from the noise source on a per data frame basis through a combining network. The passband of the received signal is then divided into a number of time-sampled data frames, with the division being dependent on the centre frequencies. The data frames have an equal bandwidth to ensure complete coverage of the frequency range. For each signal received at a wideband DF tuner, the signal is sampled within the frame. For each time-sampled data frame, multiple frequency points are chosen. For each frequency point, both an amplitude and a phase are measured for the signal received at each wideband tuner. For each given frequency point, the differences between the data collected for a wideband tuner and the data collected for another wideband tuner is stored as time-sampled data corresponding to a time-sampled data frame.
A transformation in the frequency domain, such as a Fast Fourier Transform (FFT), is performed on each set of time-sampled data. The phase difference between the signals received by the two wideband tuners is calculated for each frame at each frequency. Based on the gain and the phase differences for the frequencies sampled by the tuners, the phase and gain differences for those frequencies not sampled by the tuners which have not been previously mapped can be determined. Methods of interpolation such as best-fit curve or a polynomial curve-fitting, may be used to determine the gain and the phase difference between the two wideband tuners for frequencies not covered in the sampled data. As a result, this calibration method determines a series of calibration factors for each frame, and for the entire passband.
In a first aspect, the present invention provides a method of calibrating a sensor system of a wideband direction finder having at least two antenna elements using a noise source, the direction finder having at least two arms, each arm having a wideband tuner for receiving a noise on a per data frame basis from the noise source, with each arm coupled to the data processing means, the method including the steps of:
a) selecting a set of centre frequencies common for all wideband tuners in the direction finder;
b) dividing a passband of the noise source into a number of time-sampled data frames, such that the dividing is dependent on the centre frequencies;
c) for each time-sampled data frame received by each arm:
c1) reading a gain and a phase of the noise at each wideband tuner for a given frequency;
c2) processing the gain and the phase for each time-sampled data frame, to transform the gain and the phase into the frequency domain and to obtain intermediate results;
c3) calculating a gain difference and a phase difference between the intermediate results output from each arm;
c4) repeating steps c1) to c3) for each given frequency in a chosen group of frequencies, the chosen group of frequencies being within a frequency range of the time-sample data frame; and
c5) averaging the gain and the phase difference obtained in step c4) for the time-sampled data frame to obtain a calibration factor having a gain component and a phase component; and
d) adjusting the sensor system based on the calibration factor;
wherein a passband of the noise source covers at least one portion of a desired radio frequency band.
In a second aspect, the present invention provides a method of calibrating a sensor system of a wideband direction finder having at least two antenna elements by using a noise source, the direction finder having at least two arms, each arm having a wideband tuner for receiving a noise signal on a per data frame basis from the noise source, and an analog/digital converter, the wideband tuner being coupled to the analog/digital converter, with each arm coupled to the data processing means through the analog/digital converter, the method including the steps of:
a) coupling the noise source to the each wideband tuner in each arm;
b) selecting a set of centre frequencies common for all wideband tuners in the direction finder;
c) dividing a passband of the noise source into a number of time-sampled data frames, such that the dividing is dependent on the centre frequencies;
d) for each time-sampled data frame received by each arm:
d1) converting the noise signal into a digital noise signal for a given frequency;
d2) processing the digital noise signal to transform the digital noise signal into the frequency domain;
d3) reading a gain and a phase of a transformed digital noise signal for each time-sampled data frame to obtain intermediate results;
d4) calculating a gain difference and a phase difference between the intermediate results output from each arm;
d5) repeating steps d1) to d4) for each given frequency in a chosen group of frequencies, the chosen group of frequencies being within a frequency range of the time-sampled data frame;
d6) averaging the gain and the phase difference obtained in step d5) for the time-sampled data frame to obtain a calibration factor having a gain component and a phase component; and
e) adjusting the sensor system based on the calibration factor;
wherein a passband of the noise source covers at least one portion of a desired radio frequency band.
In a third aspect, the present invention provides a method of calibrating a sensor system of a wideband direction finder having at least two antenna elements using a noise source, the direction finder having at least two arms, each arm having a wideband tuner for receiving a noise on a per data frame basis from the noise source, with each arm coupled to the data processing means, the method including the steps of:
a) selecting a set of centre frequencies common for all wideband tuners in the direction finder;
b) dividing a passband of the noise source into a number of time-sampled data frames such that the dividing is dependent on the centre frequencies;
c) for each time-sampled data frame received by each arm:
c1) reading a gain and a phase of the noise at each wideband tuner for a given frequency;
c2) processing the gain and the phase for each time-sampled data frame to transform the gain and the phase into the frequency domain and to obtain intermediate results;
c3) calculating a gain difference and a phase difference between the intermediate results output from each arm;
c4) repeating steps c1) to c3) for each given frequency in a chosen group of frequencies, the chosen group of frequencies being within a frequency range of the time-sample data frame;
c5) interpolating the gain and the phase difference obtained in step c4) for the time-sampled data frame to obtain a calibration factor for a frequency not part of the chosen group of frequencies, the calibration factor having a gain component and a phase component; and
d) adjusting the wideband tuners based on the calibration factor;
wherein a passband of the noise source covers at least one portion of a desired radio frequency band.
In a fourth aspect, the present invention provides computer readable media having embodied thereon computer readable and computer executable instructions for a method of calibrating a sensor system of a wideband direction finder having at least two antenna elements using a noise source, the direction finder having at least two arms, each arm having a wideband tuner for receiving a noise on a per data frame basis from the noise source, with each arm coupled to the data processing means, the method including the steps of:
a) selecting a set of centre frequencies common for all wideband tuners in the direction finder;
b) dividing a passband of the noise source into a number of time-sampled data frames, such that the dividing is dependent on the centre frequencies;
c) for each time-sampled data frame received by each arm:
c1) reading a gain and a phase of the noise at each wideband tuner for a given frequency;
c2) processing the gain and the phase for each time-sampled data frame, to transform the gain and the phase into the frequency domain and to obtain intermediate results;
c3) calculating a gain difference and a phase difference between the intermediate results output from each arm;
c4) repeating steps c1) to c3) for each given frequency in a chosen group of frequencies, the chosen group of frequencies being within a frequency range of the time-sample data frame;
c5) averaging the gain and the phase difference obtained in step c4) for the time-sampled data frame to obtain a calibration factor having a gain component and a phase component; and
d) adjusting the wideband tuners based on the calibration factor;
wherein a passband of the noise source covers at least one portion of a desired radio frequency band.
In a fifth aspect, the present invention provides a computer system constructed and configured to execute computer readable and computer executable instructions having embodied therein a method of calibrating a sensor system of a wideband direction finder having at least two antenna elements using a noise source, the direction finder having at least two arms, each arm having a wideband tuner for receiving a noise on a per data frame basis from the noise source, with each arm coupled to the data processing means, the method including the steps of:
a) selecting a set of centre frequencies common for all wideband tuners in the direction finder;
b) dividing a passband of the noise source into a number of time-sampled data frames, such that the dividing is dependent on the centre frequencies;
c) for each time-sampled data frame received by each arm:
c1) reading a gain and a phase of the noise at each wideband tuner for a given frequency;
c2) processing the gain and the phase for each time-sampled data frame, to transform the gain and the phase into the frequency domain and to obtain intermediate results;
c3) calculating a gain difference and a phase difference between the intermediate results output from each arm;
c4) repeating steps c1) to c3) for each given frequency in a chosen group of frequencies, the chosen group of frequencies being within a frequency range of the time-sample data frame;
c5) averaging the gain and the phase difference obtained in step c4) for the time-sampled data frame to obtain a calibration factor having a gain component and a phase component; and
d) adjusting the wideband tuners based on the calibration factor;
wherein a passband of the noise source covers at least one portion of a desired radio frequency band.
In a sixth aspect, the present invention provides a system for calibrating a sensor system of a wideband direction finder having at least two antenna elements using a noise source, the direction finder having at least two arms, each arm having a wideband tuner for receiving a noise on a per data frame basis from the noise source, with each arm coupled to the data processing means, the system comprising:
means for selecting a set of centre frequencies common for all wideband tuners in the direction finder;
means for dividing a passband of the noise source into a number of time-sampled data frames, such that the dividing is dependent on the centre frequencies;
means for reading a gain and a phase of the noise at each wideband tuner for a given frequency;
means for processing the gain and the phase for each time-sampled data frame, to transform the gain and the phase into the frequency domain and to obtain intermediate results;
means for calculating a gain difference and a phase difference between the intermediate results output from each arm;
means for averaging the gain and the phase difference for the time-sampled data frame to obtain a calibration factor having a gain component and a phase component; and
means for adjusting the wideband tuners based on the calibration factor.
The present invention seeks to provide a method for calibrating a DF system for radio signals over a frequency range in the order of 20 MHz to 2.5Ghz. Each time-sampled frame has data that can be used to calibrate any signal received within the frequency range. The methods of interpolation or best-fit curves provide reasonably accurate calibration methods for a wideband direction finder.